San Francisco is at its best between April and September and the ideal time to visit (See below for extensive essential info if visiting then). Visiting between October and March should not be ruled out. However it should be noted that the weather during this time of year can be unpredictable.
San Francisco in the summer
It cannot be emphasized enough as to the weather conditions of San Francisco in the summer (April to September)-it is COLD! There is a mis-perception to the uninitiated that San Francisco is “suburb” of Los Angeles and less than a 3 hour drive up the coast. This is not so. San Francisco is 400 miles from LA and has a considerably different client . Summers are dry (rain is unheard of) but they can be cold and miserable for the unprepared. Foggy over cast days lead to temps that struggle to climb out of the 50s and the strong breeze coming off the bay does not help. Every summer on any average day one can spot the visitors as they are the ones in t-shirts and shorts shivering away. Luckily the cold isn’t a biting one and a solid sweatshirt, jeans and even a hat will keep things comfortable.
San Francisco in the Winter
It is during the fall and winter months (October to March) San Francisco gets its rain. Every year is different. There are many years where it rains every day in San Francisco and other years not a drop. The dry years often leave the city in temps in the 60s and 70s with no fog. No visitor should wager visiting the city that they will get dry warm day. As for the rain an average San Francisco winter sees precipitation about once a week-but as previously mentioned it can rain-sometimes torrential, everyday all the way through to early April .